The present invention relates to picture processing apparatus, and more particularly to a picture processing apparatus with texture elimination capability in a facsimile, which facilitates reproduction of a high quality picture from a document containing a character region and a continuous/screened graphic region.
Conventionally, some picture processing methods have been proposed which reproduce high quality, precise images from original images by a picture processing apparatus in a facsimile. For example, Japanese Laid-Open Patent Application No. 63-169174 discloses a conventional picture processing method in which it is determined whether each of a set of image data from a document is included in a non-edge portion of the the document or in an edge portion thereof. When the image data are determined as being included in a non-edge portion of the original image, weights are periodically applied to the image data and then a binary coding is performed by using a so-called error diffusion method.
This conventional picture processing method described above is achieved by a picture processing apparatus which is constructed as shown in FIG. 1. The picture processing apparatus as shown in FIG. 1 generally has an image sensor part 21, an A/D converter 22, a correction circuit 23, an edge detection circuit 25, a multilevel dither matrix circuit 24, a binarization circuit 26 and a printer 27. The image sensor part 21 scans a document to input image signals indicative of picture elements, or pixels, in the document being scanned. The A/D converter 22 converts each of the image signals into a digital data, and the correction circuit 23 corrects the digital data from the A/D converter 22 to eliminate image defects related to the sensitivity of image data supplied by the image sensor part 21. The edge detection circuit 24 determines whether each image data is located in an edge portion of the original images or in a non-edge portion thereof. The multilevel dither matrix circuit 24 serves to perform conversion or quantization of the image data. The multilevel dither matrix circuit 24 uses, for example, seven 3.times.3 dither matrixes to convert 8-bit data into 3-bit data. The binarization circuit 26 binarizes the image data from the correction circuit 23 through the error diffusion method. The printer 27 outputs the image data from the binarization circuit 26. When image data is detected by the edge detection circuit 25 as being included in a non-edge portion of the original image, the image data is supplied from the multilevel dither matrix circuit 24 to the binarization circuit 26, so that the image data is binary-coded through the error diffusion process. With some dither matrixes of a concentrated dot type in the multilevel dither matrix circuit 24 being used, the picture processing apparatus can form pseudo screened dots from the image data when the binarization circuit 26 performs binary coding of the image data through the error diffusion method. Advantageous features of the above mentioned picture processing apparatus using dither matrixes of a concentrated dot type are that weights given to image data to which data in the midst of the dither matrix are applied are greater than weights given to image data to which data in the circumference of the dither matrix are applied, and undesired stripe patterns or noise-causing spots may be reduced in reproduced images.
However, the above conventional apparatus is not able to reproduce a high quality, precise image for the whole of a character of an original document when image data from the original document are binary-coded through the error diffusion method. The conventional apparatus can merely prevent the reproduced images of edge portions of characters from deteriorating. For example, the edge detection circuit may determine erroneoutly that image data corresponding to the central part of a boldface line in a character is included in a non-edge portion of the character, and weights are assigned to such image data, thus the reproduced image of the character becoming unclear or being of poor quality after such image data is binarized through the error diffusion method.
In addition, in the present invention, some improved picture processing methods for eliminating undesired texture from reproduced images are disclosed, the undesired texture appearing due to the error diffusion method being used. In one of the improved methods, weights are periodically applied to image data at regular intervals therebetween. In another improved method, threshold values used for binary coding of multilevel digital data through the error diffusion process performed by the binarization circuit are varied periodically.